1. Field of the Invention
The present invention relates to a process for the preparation of epoxides from lower alkane-1,2-diols. More particularly, the invention relates to a process for dehydrating ethylene glycol or propylene glycol to give the corresponding oxide, (i) in the liquid phase, (ii) at a pressure which is equal to or greater than atmospheric, and (iii) in the presence of a least one high-boiling carboxylic acid and at least one alkali metal or alkaline earth metal carboxylate.
2. Description of the Prior Art
It is well known to the art that epoxides such as ethylene oxide or propylene oxide are conventionally prepared via the chlorohydrin route. However, this process has the disadvantage in that it produces polluting effluents.
Methods are also known for preparing epoxides by oxidizing olefins, especially with molecular oxygen or the organic hydroperoxides. However, in the oxidation by means of oxygen, acids are formed, in particular formic acid, which readily react with the epoxides to give undesirable by-products; these secondary reactions occur notably in that stage when the epoxides are separated from the reaction medium. In the oxidation with hydroperoxides, the alcohol which corresponds to the hydroperoxide employed, and the utilization of which is frequently uncertain, is obtained in addition to the desired epoxide.
Methods for producing epoxides have also been described, which consist of decomposing either hydroxyesters of organic acids in the vapor phase (French patent application No. 2,224,454, dated Mar. 8, 1974, and U.S. Pat. No. 4,012,424, both assigned to Chem Systems Inc.), or cyclic esters obtained from alkane-1,2-diols and from compounds such as phosgene or sulfuryl chloride (German patent application No. 1,940,205, dated Aug. 7, 1968, assigned to Farbwerke Hoechst), into epoxides and acids (or anhydrides of inorganic acids).
In the first proposed process, the difficulty lies in the route to the monoacetate, because vicinal hydroxyesters tend to disproportionate into .alpha.-glycol and diacetate, at the high temperatures required to carry out the said process.
The second proposed process leads to the consumption of a co-reactant, and the evolution of carbon dioxide gas. Therefore, this process cannot logically be considered for industrial purposes.
In addition, the formation of large amounts of acids poses corrosion problems in both proposed processes.
Furthermore, there is an abundance of literature articles on the subject of dehydration of diols. In general terms, this literature teaches that epoxides or cyclic ethers can be produced by dehydrating diols which contain an aliphatic ring or one or more aromatic nuclei in their molecules, or diols which belong to the wholly aliphatic series and in which the hydroxyl groups are not located on adjacent carbon atoms, but that, on the other hand, in the case of vicinal aliphatic diols, and especially in the case of the 1,2-diols, no epoxides are produced, the dehydration preferentially yielding aldehydes and ketones.
The only pertinent information to be gleaned from the prior art is the formation of propylene oxide as a by-product, in addition to the propionaldehyde obtained as the principal product, during the dehydration of propylene glycol over catalysts based on alumina, silica and copper or nickel oxide [Chemical Abstracts. 54, 10,844 g (1960)] and the production of an epoxide by dehydrating cyclodecanediol or another heavy diol, which was, moreover, not completely defined, namely, a 2-alkoxy-2,6-dimethyloctane-7,8-diol, in the presence of m-toluic acid and a trace quantity of sodium acetate. [H. R. Ansari and R. Clark, Tetrahedron Letters, No. 35, pages 3,085-3, 3,086 (1975)]. However, according to experiments conducted by the assignee thereof, application of the latter method to propylene glycol does not provide any appreciable amount of propylene oxide.